Copper . 



. . + 0-40 volt, 



Silver . . 



. o-oo „ 



Gold 



— 0-2 



258 Sig. Quirino Majorana on the Contact Theory. 



any attraction if the current be now established as before, but 

 attraction occurs if the current passes from M to N. 



From the preceding experiments we obtain a simple and 

 rapid method of measuring the electromotive force of contact 

 of two metals, or, better, of any metal and silver ; we have 

 only to observe what electromotive force suffices to annul the 

 attractiou. Although at first sight the method may not appear 

 to be capable of great precision, because the effect observed 

 is so small, and the condition of the surface of silver on the 

 fibre is somewhat uncertain, yet T have been able without 

 difficulty to arrange several metals in a series as follows : — 



Aluminium ... + 1*1 volt. 



Zinc + 0-9 „ 



Iron + 05 ,, 



Brass -+- 0*45 „ 



It is remarkable that I have found gold to be always slightly 

 negative to silver ; Pellat, on the other hand, puts gold and 

 silver together in the Volta series. 



Lord Kelvin has pointed out that the attraction which 

 should exist between* two disks of different metals occurs 

 when they are metallically connected. Now this, although 

 true, is not always necessary in practice. Let us suppose the 

 two disks to be each connected with a metallic conductor of 

 large capacity which has been discharged to earth and then 

 insulated. It is evident that we may obtain attraction in this 

 case also, because on bringing the disks near each other the 

 two capacities will supply the quantity of electricity necessary 

 to maintain the disks at the same potential. In the experi- 

 ment described the action is somewhat similar. Attraction 

 between the zinc plate and the silvered fibre does, in fact, 

 occur even when they are insulated ; this must be so, since 

 the attracting portions are very small in comparison with 

 the rest. 



It is a very difficult matter to calculate a priori the value of 

 the attractive force in the experiment described. If we have, 

 instead, two disks of equal area A, placed at a distance apart 

 D which is small in comparison with their diameter, the 

 potential-difference between them being V, it is easily seen 

 that the attraction between them is 



~~ 8ttD 2 

 In order to obtain an idea of the magnitude of the forces 



